Facing stringent regulations on wastewater discharge containing heavy metal ions, various industries are demanding more efficient and effective treatment methods. Among the methods available, nanofiltration (NF) is a feasible and promising option. However, the development of new membrane materials is constantly required for the advancement of this technology. This is a report of the first attempt to develop a composite NF membrane comprising a molecularly designed pentablock copolymer selective layer for the removal of heavy metal ions. The resultant NF membrane has a mean effective pore diameter of 0.50 nm, a molecular weight cutoff of 255 Da, and a reasonably high pure water permeability (A) of 2.4 LMH/bar. The newly developed NF membrane can effectively remove heavy metal cations such as Pb(2+), Cd(2+), Zn(2+), and Ni(2+) with a rejection of >98.0%. On the other hand, the membrane also shows reasonably high rejections toward anions such as HAsO4(2-) (99.9%) and HCrO4(-) (92.3%). This performance can be attributed to (1) the pentablock copolymer's unique ability to form a continuous water transport passageway with a defined pore size and (2) the incorporation of polyethylenimine as a gutter layer between the selective layer and the substrate. To the best of our knowledge, this is the first reported NF membrane comprising this pentablock copolymer as the selective material. The promising preliminary results achieved in this study provide a useful platform for the development of new NF membranes for heavy metal removal.